Akuma has the killing need. He's not evil, just fights to the death. So he's technically more powerful. The whole 'evil' ryu thing was a what if scenario for Ryu to have the same need to kill his opponents.

In the alpha series it explains that between SF 1 and SF A2 Ryu had the fight till the death need. That's how Sagat got the scar on his chest. And for a while there Ryu was tipping between being fighting cleanly and fighting to kill. Sagat had went to offer his hand to ryu when ryu did the shoryuken. A dirty fight move since Sagat had basically won and had assumed Ryu needed help.

Ryu talked to Akuma and Akuma told him that he just needed to want to kill in order to be as powerful as himself. Ryu refused, because when he went to kill Bison (Vega jp) he was controlled by psycho power. Cammy and Ken rescued Ryu, and Sagat saw the psycho power generators for what they were and destroyed them. And Psycho power is a sister power to that which Akuma weilds, so ryu could turn him down.

Sagat had wanted revenge. But he remembered a fight where he got revenge on a Muay Thai fighter who took out his eye, and killed him. So he saw the stupidity of that way and never went there. Later he met Dan who's father was the Muay Thai figher Sagat had killed. Dan wanted revenge. Sagat threw the fight so that Dan felt avenged. Sagat is a good guy in the SF universe now.